Refrigeration systems having prescriptive refrigerant flow control

ABSTRACT

A refrigeration system for a frozen product dispenser is controlled to have a variable cooling capacity that is determined by variable cooling load demands of the dispenser. This is accomplished, in part, by providing the refrigeration system with a variable speed compressor and one or more adjustable expansion valves for metering refrigerant to associated evaporators that are heat exchange coupled to associated freeze barrels of the dispenser, and by controlling the metering setting of the expansion valves and the speed of operation of the compressor in accordance with the cooling load demands of the dispenser. The arrangement provides for efficient operation of the refrigeration system from an energy standpoint and for a reduction in on/off cycling of the system.

This application claims benefit of provisional application Ser. No.60/857,625, filed Nov. 8, 2006.

FIELD OF THE INVENTION

The present invention relates to refrigeration systems, and inparticular to a variable capacity refrigeration system for efficientlyhandling large variations in cooling load requirements.

BACKGROUND OF THE INVENTION

Depending upon the nature of a load that a refrigeration system isrequired to service, cooling load requirements imposed on therefrigeration system by the load can vary widely, such for example aswhen the refrigeration system serves the cooling requirements a frozenbeverage product dispenser. In the case of a frozen beverage productdispenser, customer demand for beverages can vary from no drinksdispensed per minute to as many as 3 or 4 or more drinks served perminute. This volatile variation in customer demand results in a verybroad range in cooling load requirements for a refrigeration system of atypical frozen product dispenser, for example as is shown by the chartof FIG. 9. As can be seen, depending upon ambient temperature and duringperiods when no product is being drawn, the maintenance cooling load ofa frozen product dispenser can be as low as about 1500 Btu/hr. At theother extreme and during periods of high drink draw rates, for examplewhen delivering drinks at the rate of 4×16 oz drinks per minute, coolingload requirements of a frozen product dispenser may be in excess of18,000 Btu/hr. This represents about a 12:1 turndown ratio, which froman energy standpoint conventional refrigeration systems are not able toefficiently accommodate.

As is known, refrigeration systems of conventional frozen productdispensers utilize a compressor that delivers refrigerant through acondenser to one or more expansion valves, each of which deliversrefrigerant to an associated evaporator coil that is heat transfercoupled to an associated beverage product freeze barrel in order tochill the barrel and at least partially freeze beverage product in thebarrel. To accommodate various cooling load requirements of the barrels,the expansion valves are variably controlled to meter refrigerant atvarious flow rates to the evaporators. As load requirements of anevaporator coil change due to changing customer demands, the expansionvalve supplying refrigerant to the evaporator changes to a moreappropriate flow metering position. The objective is to adjust theexpansion valve so as to match the cooling capability of the evaporator,based upon refrigerant flow to the evaporator, more closely to thedynamically changing cooling load requirements of the barrel beingchilled by the evaporator. However, since customer demand for frozenbeverage product can vary from no drinks served per minute to as many as3 or 4 or more drinks served per minute, it is not practical to relyupon a typical control system loop using temperature sensors forfeedback. Temperature sensors are too slow to respond to a need forchilling product, since they have a 1^(st) order response time on theorder of 5 seconds or more, and meanwhile there is a cooling load thatis changing faster than the temperature sensors can respond to and thecontrol system cannot keep up with the changing cooling loadrequirements. In addition, a fixed speed compressor, as is normally usedfor a frozen product dispenser, is not readily able to accommodatechanges in cooling load requirements and is best suited to providingrefrigerant flow at a certain constant rate, despite changes that may beoccurring in the cooling load that could best be served by varying theflow rate of refrigerant from the compressor. Consequently,refrigeration system balance becomes disturbed as the expansion valvesare frequently adjusted in an attempt to meet changing cooling loadrequirements, resulting in saturated evaporator temperatures dropping ascooling load requirements decrease, rising as cooling load requirementsincrease, and overall poor control over the temperature of theevaporator. In addition, because the compressor operates at a constantspeed, when cooling load requirements decrease, cooling of beverageproduct in the barrel is quickly satisfied and the compressor must befrequently cycled off/on, resulting in increased stress of compressorcomponents. As a result, where the compressor is not matched with thecooling load, during periods of low product demand the compressor willcycle on/off excessively and the system will operate less efficientlyand use more energy than is required.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a prescriptiverefrigerant flow control scheme for operating a refrigeration system ina manner such that the refrigeration system efficiently and quicklyresponds to dynamically changing wide variations in cooling loadrequirements.

Another object is to provide such a prescriptive refrigerant flowcontrol scheme for operating a refrigeration system having a variablespeed compressor, such that the speed of the compressor is controlled inaccordance with the cooling load to be satisfied.

A further object is to provide such a prescriptive refrigerant flowcontrol scheme that variably controls the metering position ofevaporator expansion valves of the refrigeration system in accordancewith the cooling load.

Yet another object is to provide such a prescriptive refrigerant flowcontrol scheme that measures a cooling load to be satisfied and thenoperates the refrigeration system to provide a total Btu output inaccordance with the cooling load.

SUMMARY OF THE INVENTION

In accordance with the present invention, a refrigeration systemcomprises an evaporator for being heat exchange coupled to a load to becooled; an expansion valve adjustable to meter refrigerant to theevaporator at various rates; a variable speed compressor controllable todeliver refrigerant to the expansion valve at various rates inaccordance with its speed of operation; and means for operating therefrigeration system to adjust the expansion valve to meter refrigerantto the evaporator at a rate, and to control the speed of operation ofthe compressor to deliver refrigerant to the expansion valve at a rate,determined in accordance with a cooling load requirement of the load.

In a contemplated embodiment of the refrigeration system, the means foroperating adjusts the expansion valve and controls the compressor suchthat the refrigeration system provides a selected cooling output to theevaporator within a selected time period.

The means for operating can comprise means for sensing the coolingrequirements of the load; means responsive to the means for sensing fordetermining the mass flow of refrigerant through the expansion valvenecessary for the evaporator to at least closely satisfy the coolingrequirements of the load; means for determining the flow rate ofrefrigerant through the expansion valve in order to meter the determinedmass flow of refrigerant through the expansion valve to the evaporatorwithin a selected time; means for adjusting the expansion valve to meterthe determined flow rate of refrigerant to the evaporator; and means foroperating the compressor at a speed selected to deliver the determinedflow rate of refrigerant to the expansion valve.

The load can be a frozen beverage product dispenser having a freezebarrel for being heat transfer coupled to the evaporator. The barrelreceives liquid beverage product to be cooled by the evaporator tofreeze the beverage product in the barrel, and the means for sensing thecooling requirements of the load comprises means for sensing the volumeand temperature of beverage product delivered into the freeze, togetherwith means responsive to the means for sensing for determining thenumber of Btu's of cooling required to be provided to the barrel tofreeze the beverage product in the barrel.

The invention also contemplates a method of operating a refrigerationsystem, comprising the steps of heat exchange coupling an evaporator toa load to be cooled; flowing refrigerant to the evaporator through anexpansion valve that is adjustable to meter refrigerant to theevaporator at various rates; delivering refrigerant to the expansionvalve with a compressor having an operating speed that is adjustable todeliver refrigerant to the expansion valve at various rates; andoperating the refrigeration system to adjust the expansion valve tometer refrigerant to the evaporator at a rate, and to adjust the speedof operation of the compressor to deliver refrigerant to the expansionvalve at a rate, determined in accordance with a cooling loadrequirement of the load.

The operating step can adjust the expansion valve and the compressorsuch that the refrigeration system provides a selected cooling output tothe load in a selected time period, and may comprise the steps ofsensing the cooling requirements of the load; determining, in responseto the sensing step, the mass flow of refrigerant through the expansionvalve necessary for the evaporator to at least closely satisfy thecooling requirements of the load; determining the flow rate ofrefrigerant through the expansion valve in order to meter the determinedmass flow of refrigerant through the expansion valve to the evaporatorwithin a selected time; adjusting the expansion valve to meter thedetermined flow rate of refrigerant to the evaporator; and operating thecompressor at a speed selected to deliver the determined flow rate ofrefrigerant to the expansion valve.

In a contemplated practice of the method, the load is a frozen beverageproduct dispenser having a freeze barrel for being heat transfer coupledto and for being cooled by the evaporator and that receives liquidbeverage product to be frozen in the barrel, and the step of sensing thecooling requirements of the load comprises sensing the volume andtemperature of beverage product delivered into the barrel: anddetermining, in response to the sensing step, the number of Btu's ofcooling required to freeze the beverage product in the freeze barrel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one type of variable capacityrefrigeration system that may be operated according to a prescriptiverefrigerant flow control scheme of the invention, which is adapted foruse in a frozen product dispenser for chilling two product freezebarrels and a product pre-chiller of the dispenser;

FIG. 2 is similar to the system of FIG. 1, except that the refrigerationsystem does not include a pre-chiller;

FIG. 3 is a schematic representation of one type of frozen beverageproduct dispensing system utilizing ambient temperature carbonation, ofa type with which a variable capacity refrigeration system may be used;

FIG. 4 is a schematic representation of another type of frozen beverageproduct dispensing system utilizing chilled carbonation, of another typewith which a variable capacity refrigeration system may be used;

FIG. 5 is a schematic representation of a further type of frozenbeverage product dispensing system utilizing an in-line chilledcarbonation system, of a further type with which a variable capacityrefrigeration system may be used;

FIGS. 6-8 are control strategy and function tables of contemplatedmanners of operation of the variable capacity refrigeration system ofFIG. 1;

FIG. 9 is a chart showing typical cooling load requirements for a frozencarbonated beverage (FCB) dispenser at various product draw rates andambient temperatures;

FIG. 10 is a chart showing representative cooling load requirements foran FCB dispenser at an ambient temperature of 75° F. for various productdraw rates in each of the three operational modes of maintaining,pre-chilling and freezing beverage product;

FIG. 11 is a chart similar to the one of FIG. 10, except that theambient temperature is 90° F.;

FIG. 12 is a control strategy and function table, showing a contemplatedmanner of operation of the variable speed compressor of therefrigeration system of FIG. 1 in response to various FCB machinecooling loads;

FIG. 13 is a graph showing various speeds of operation of therefrigeration system compressor in accordance with the number of frozenbeverages served per minute;

FIG. 14 is a graph showing refrigerant mass flow metered through astepper motor controlled expansion valve for various ambienttemperatures and positions of the valve;

FIG. 15 is a graph showing refrigerant mass flow through the expansionvalves for various ambient temperatures and beverage draw rates;

FIG. 16 is a graph showing refrigerant mass flow versus brix valve ontime for various ambient temperatures;

FIG. 17 is a table showing expansion valve metering positions forvarious modes of operation of an FCB dispenser;

FIG. 18 is a schematic of a prescriptive refrigerant flow controlstrategy for the refrigeration system expansion valves;

FIGS. 19A and 19B show a microprocessor control for controlling an FCBdispenser in accordance with the prescriptive refrigerant flow controlscheme of the invention; and

FIG. 20 is a flow chart illustrating the prescriptive refrigerant flowcontrol scheme of the invention.

DETAILED DESCRIPTION

The invention provides a novel prescriptive refrigerant flow controlscheme for operating a variable capacity refrigeration system, such thatthe cooling capacity of the refrigeration system is closely matched to awide range of cooling loads to be satisfied by the system. Toefficiently provide various cooling capacities, the refrigeration systemutilizes both a variable capacity compressor that is driven at variousselected speeds, along with adjustable expansion valves that arecontrolled to meter refrigerant flows to associated evaporators atvarious selected rates. The arrangement is such that the compressor isoperated at speeds selected, and the expansion valves are operated tometer refrigerant at flows selected, to provide refrigerant flows to theevaporators such that the refrigeration system provides a coolingoutput, over a selected cycle time, that closely a matches cooling loadto be satisfied. While a refrigeration system operated according to theprescriptive refrigerant flow control scheme may advantageously be usedin various diverse applications where dynamically changing cooling loadrequirements are encountered, a presently contemplated use for such arefrigeration system is in providing cooling for a frozen productdispenser, such as a frozen carbonated beverage (FCB) dispenser, and theinvention will therefore be described in that environment.

It is desirable that a refrigeration system for an FCB dispenser be ableto handle a broad range of cooling loads imposed upon it, in order thatthe dispenser might maintain good control over frozen beverage producttemperature and viscosity. Unlike conventional refrigeration systems forFCB dispensers, which use a fixed speed compressor that is sized for amaximum load situation, in practice of the invention the refrigerationsystem has both a variable speed compressor and adjustable evaporatorexpansion valves that are controlled to meter refrigerant to associatedevaporators in accordance with refrigerant flows provided by thecompressor. The arrangement enables the refrigeration system, whenoperated according to the prescriptive refrigerant flow control schemeof the invention, to have its cooling capacity closely matched to thecooling load requirement of the FCB dispenser at any point in real time.In general, the pumping rate of the compressor and the refrigerant flowsthrough the evaporator expansion valves are decreased when cooling loadsdecrease and increased when cooling loads increase, in a manner tomaintain high refrigeration system efficiency. It is contemplated thatthe variable speed compressor have, preferably but not necessarily, aspeed range of at least 3:1, for example about 50% nominal speed atminimum cooling capacity to as much as 150% nominal speed at maximumcooling capacity, which provides the ability to closely and efficientlymatch compressor cooling capacity to dispenser cooling load requirementsover a fairly broad range. As a result, the need for the compressor tocycle off/on is significantly reduced, which significantly reduces thefrequency of startup stresses on the compressor.

Some of the benefits achieved in use of the prescriptive refrigerantflow control scheme of the invention include improvements inrefrigeration cycle and energy efficiency because of a better matchingof compressor pumping rate and evaporator expansion valve refrigerantflow rates to dispenser cooling load requirements; improvements in thereliability of the compressor because of reduced on/off cycling;improvements in the consistency of the temperature and viscosity offinished frozen beverage product because of better control over coolingcapacity; a reduction in the noise levels of the refrigeration system,since the compressor will often run at lower speeds; and a furtherdecrease in operating noise as a result of a reduction in condenser fanspeed as compressor speed is reduced.

Referring to the drawings, a refrigeration system that may be operatedaccording to the prescriptive refrigerant control scheme of theinvention is shown in FIG. 1 and indicated generally at 20. Therefrigeration system includes a variable speed/capacity compressor 22,which may be a scroll or a reciprocating compressor that has avariable-frequency drive for applying to an ac motor of the compressoran ac voltage signal that has a frequency selected to provide a desiredspeed of operation of the motor and, thereby, a desired refrigerant flowrate from and output capacity of the compressor. Hot refrigerant at anoutlet from the compressor is coupled through a refrigerant line 24 toan inlet to a condenser 26, through which air is drawn by a fan 28 tocool the refrigerant. Cooled refrigerant at an outlet from the condenserflows through a refrigerant line 30 to and through a filter/dryer 32 anda refrigerant line 34 to inlets to each of three electronicallycontrolled adjustable expansion valves 36, 38 and 40 that may be of thestepper motor driven or pulse valve modulated type, such that the valvesmay be controlled to meter selected refrigerant flows from theiroutlets. Refrigerant exiting an outlet from the expansion valve 36 isdelivered to an inlet to an evaporator coil 42 that is heat transfercoupled to a first beverage product freeze barrel 44 of an FCB dispenserto chill the barrel and freeze beverage product in the barrel.Refrigerant exiting an outlet from the expansion valve 38 is deliveredto an inlet to an evaporator coil 46 that is heat transfer coupled to asecond beverage product freeze barrel 48 of the dispenser to chill thebarrel and freeze beverage product in the barrel. Refrigerant exiting anoutlet from the expansion valve 40 is delivered to an inlet to anevaporator coil 50 that is heat transfer coupled to a pre-chiller 52 ofthe dispenser to chill the pre-cooler and, thereby, to chill beverageproduct flowed through the pre-chiller before being introduced into thebarrels 44 and 48. After passing through each of the barrel evaporators42 and 46, refrigerant exiting outlets from the evaporators flowsthrough a refrigerant line 54 and an accumulator 56 to an inlet to thecompressor 22. After passing through the pre-cooler evaporator 50,refrigerant exiting the evaporator flows through an evaporator pressureregulating valve 58 and then through the refrigerant line 54 andaccumulator 56 to the inlet to the compressor. The evaporator pressureregulating valve 58 serves to prevent the pressure of refrigerant in theevaporator 50 from falling below a lower limit, thereby to preventfreezing of beverage product in the pre-chiller 52.

The refrigeration system 20 has two defrost circuits, a first one ofwhich includes a solenoid operated refrigerant valve 60 having an inletcoupled to hot refrigerant at the outlet from the compressor 22 througha refrigerant line 62 and an outlet coupled to the inlet to the freezebarrel evaporator 42 through a refrigerant line 64. A second defrostcircuit includes a solenoid operated refrigerant valve 66 having aninlet coupled to hot refrigerant at the outlet from the compressor 22through a refrigerant line 68 and an outlet coupled to the inlet to thefreeze barrel evaporator 46 through a refrigerant line 70. The defrostcircuits are operated to heat the evaporators 42 and 46 to defrost thebeverage product barrels 44 and 48 in defrost cycles of therefrigeration system.

The refrigeration system 20 is adapted for use with an FCB dispenserthat has a pre-chiller 52. To provide chilling for FCB dispensers thatdoes not have a pre-chiller, a refrigeration system of a type shown inFIG. 2 and indicated generally at 72 may be used. The refrigerationsystem 72 is similar to the refrigeration system 20, and like referencenumerals have been used to denote like components. A difference betweenthe two systems is that since the system 72 does not provide for coolingof a pre-chiller 52, it does not have an evaporator coil 50, anelectronically controlled expansion valve 40 and an evaporator pressureregulating valve 58. Otherwise, the structure and operation of the tworefrigeration systems 20 and 72 is generally similar.

Since operation of a refrigeration system according to the prescriptiverefrigerant control scheme of the invention, to provide cooling for anFCB dispenser having a pre-chiller, generally embodies operation of arefrigeration system to provide cooling for an FCB dispenser that doesnot have a pre-chiller, the invention will be described in terms of therefrigeration system 20 being used with an FCB dispenser having apre-chiller. One such FCB dispenser is shown in FIG. 3 and indicatedgenerally at 80, and includes the two beverage product freeze barrels 44and 48, only the barrel 44 being shown. The dispenser utilizes ambienttemperature carbonation, and while not specifically shown in FIG. 3 (butshown in FIG. 1), it is understood that the evaporator coil 42 is heattransfer coupled to the barrel 44 to chill the barrel in order to freezea beverage product mixture delivered into the barrel. With reference tothe portion of the dispenser 80 shown and associated with the freezebarrel 44, it being understood that a like description would apply to asimilar but less than fully shown portion of the dispenser associatedwith the freeze barrel 48, a frozen beverage product dispensing valve 82is coupled to the barrel 44 for service of frozen beverages. To deliverliquid beverage components to the barrel 44 for being frozen, anexternally pumped beverage syrup concentrate is delivered to an inlet toa syrup brixing valve 84 through a syrup line 85, to which line iscoupled a sensor 86 for detecting a syrup-out condition. To deliverliquid beverage components to the barrel 48 (shown in FIG. 1), anexternally pumped beverage syrup concentrate is delivered to an inlet toa syrup brixing valve 87 through a syrup line 88, to which line iscoupled a sensor 89 for detecting a syrup-out condition. A potable watersupply, such as from a city main, is connected to the dispenser througha strainer/pressure regulator 92 to which is coupled a pressure switch94 for detecting a water-out condition, and from the strainer/pressureregulator the water passes through a carbonator pump 96 and a checkvalve 98 to a water inlet to a carbonator 100. The carbonator 100operates in a manner well understood in the art to carbonate water, andcarbonated water at an outlet from the carbonator is delivered to aninlet to a water brixing valve 102 associated with the syrup brixingvalve 84, and to an inlet to a water brixing valve 104 associated withthe syrup brixing valve 87. The brixing valves 104, 87 comprise anassociated pair of brixing valves that deliver a water and syrupmixture, in a selected ratio, through a fluid circuit (not shown) thatincludes the pre-chiller 52, to the freeze barrel 48. The brixing valves102, 84 comprise an associated pair of brixing valves that deliver awater and syrup mixture, in a selected ratio, through an associatedfluid circuit that includes the pre-chiller 52, to and into the freezebarrel 44. The water/syrup beverage mixture provided at an outlet fromeach pair of brixing valves is in a ratio determined by the settings ofthe individual valves of each pair. The water and syrup mixturedelivered from the pair of brixing valves 102, 84 is delivered through a3-way valve 106 and the pre-chiller 52 to the freeze cylinder or barrel44, it being understood that, although not shown (but shown in FIG. 1),the evaporator coil 50 is heat exchange coupled to the pre-chiller. The3-way valve 106 has an outlet 108 leading to atmosphere, by means ofwhich a sample of the water and syrup mixture output by the pair ofbrixing valves 102 and 84 may be collected for analysis, so that anynecessary adjustments may be made to the brixing valves to provide adesired water/syrup ratio.

To carbonate water in the carbonator tank 100, an externally regulatedsupply of CO₂ is coupled through a temperature compensated pressureregulator 110 and a check valve 112 to the carbonator, the regulator 110including a capillary sensor 114 for detecting the temperature ofincoming water. A sensor 116 detects a CO₂-out condition, and the supplyof CO₂ is also coupled to inlets to each of two CO₂ pressure regulatorsof a manifold 118. An outlet from a first one of the manifold pressureregulators is coupled through a solenoid shut-off valve 119, a CO₂ flowcontrol valve 120 and a CO₂ check valve 121 to the water and syrupmixture line extending between the pre-chiller 52 and an inlet to thefreeze barrel 44. In addition, CO₂ at an outlet from the second one ofthe manifold pressure regulators is coupled to an upper opening to anexpansion tank 122, a lower opening to which is coupled to the water andsyrup mixture line between the pre-chiller and freeze barrel. The flowcontrol valve 120 accommodates adjustment of the carbonation level inthe barrel 44 by enabling the introduction of CO₂ into the barrel for abrief period of time before a mixture of water and syrup is deliveredinto the barrel. A pressure transducer 124 monitors the pressure of thewater and syrup mixture delivered to and into the barrel 44 and servesas a pressure cut-in/cut-out sensor to control filling and refilling ofthe barrel with beverage product to be frozen. As is understood by thoseskilled in the art, when the pressure transducer 124 detects a lowerlimit cut-in pressure in the barrel, for example 23 psi, the pair ofbrixing valves 102, 84 is opened for flow of a water and syrup mixtureto and into the barrel to refill the barrel, until the pressuretransducer detects an upper limit cut-out pressure, for example 29 psi,whereupon the pair of brixing valves is closed. During flow of the waterand syrup mixture to the barrel, the mixture is cooled as it flowsthrough an associated circuit in the pre-chiller 52. During freezing ofthe mixed beverage product in the freeze barrel 44, the mixture expandsand the expansion chamber 122 provides a volume for a portion of theexpanding mixture to move into.

As mentioned, the dispenser 80 includes the freeze barrel 48 and,therefore, includes further structure (not shown) that is generallyduplicative of that to the right of the pair of brixing valves 102, 84and that accommodates delivery of a water and syrup mixture from thepair of brixing valves 104, 87 to the barrel 48, except that thebeverage mixture does not flow through a separate pre-chiller, butinstead flows through an associated circuit of the pre-chiller 52. Inaddition, a line 126 delivers CO₂ to an upper opening to an expansionchamber (not shown), a lower opening from which is coupled to an inletto the barrel 48, and to accommodate addition of CO₂ to the barrel 48,the outlet from the manifold first CO₂ pressure regulator is alsocoupled through a solenoid shut-off valve 128, a CO₂ flow control valve130 and a CO₂ check valve 132 to the inlet to the barrel.

Another type of FCB dispenser with which the refrigeration system 20 maybe used and operated according to the prescriptive refrigerant flowcontrol scheme of the invention, is shown in FIG. 4 and indicatedgenerally at 140. The dispenser 140 is somewhat similar to the FCBdispenser 80 of FIG. 3, except that it utilizes chilled carbonation, andlike reference numerals have therefore been used to denote likecomponents. With reference to the portion of the dispenser 140associated with the freeze barrel 44, it being understood that a similardescription applies to a similar but only partially shown structure ofthe dispenser associated with the freeze barrel 48, to deliver liquidbeverage components to the barrel 44 for being frozen, an externallypumped beverage syrup concentrate is delivered to the syrup brixingvalve 84 through the syrup line 85, to which is coupled the sensor 86for detecting a syrup-out condition. To deliver beverage components tothe barrel 48, an externally pumped beverage syrup concentrate isdelivered to the inlet to the syrup brixing valve 87 through the syrupline 88, to which is coupled the sensor 89 for detecting a syrup-outcondition. A potable water supply is connected to the dispenser througha strainer/pressure regulator 92, to which is coupled a pressure switch94 for detecting a water-out condition. The outlet from thestrainer/pressure regulator 92 is coupled to an inlet to a CO₂ drivenwater pump 96, and unlike the dispenser 80 of FIG. 3, in which theoutlet from the water pump is delivered to an inlet to an ambienttemperature carbonator 100, in the FCB dispenser 140, an outlet from thewater pump 96 is fluid coupled directly to the inlet to each of thewater brixing valves 102 and 104. The brixing valves 104, 87 deliver awater/syrup mixture in a selected ratio through an associated fluidcircuit (not shown) that includes the pre-chiller 52 to the freezebarrel 48, and the brixing valves 102, 84 deliver a water/syrup mixturein a selected ratio through the 3-way valve 106 and the pre-chiller 52to an inlet to the freeze barrel 44. The outlet 108 from the valve 106provides a means by which a sample of the water/syrup mixture flowedfrom the brixing valves 102, 84 may be collected for analysis, such asby means of a hygrometer reading, so that any necessary adjustments maybe made to the brixing valves to provide the desired water/syrup ratio.

An externally regulated CO₂ supply is coupled through a line 136 toinlets to each of four CO₂ pressure regulators of a manifold 134, towhich line is coupled the sensor 116 for detecting a CO₂-out condition.An outlet from a first one of the manifold pressure regulators iscoupled through a line 138 to the CO₂ driven water pump 96 to operatethe pump. An outlet from a second one of the manifold CO₂ pressureregulators is coupled through the solenoid shut-off valve 119, the CO₂flow control valve 120 and the CO₂ check valve 121 to the chilledwater/syrup mixture flowing from the pre-chiller 52 to the inlet to thefreeze barrel 44, thereby to selectively carbonate the chilled beveragemixture in accordance with the solenoid shut-off valve 119 being open orclosed and the setting of the flow control valve 120, whereby eithercarbonated or non-carbonated beverages may selectively be frozen in thebarrel 44. An outlet from a third one of the manifold CO₂ pressureregulators is coupled to the upper opening to the expansion tank 122,the lower opening to which is coupled to the water/syrup mixture lineextending between the outlet from the pre-chiller 52 and inlet to thefreeze barrel 44. When frozen carbonated beverages are served, the flowcontrol valve 120 accommodates adjustment of the carbonation level inthe barrel 44. The pressure transducer 124 monitors the pressure of thebeverage mixture introduced into and within the barrel and serves as acut-in/cut-out pressure sensor control refilling of the barrel. Whenbeverage product is frozen in the barrel it expands and the expansiontank 122 accommodates such expansion by receiving some of the beverageproduct.

Since the dispenser 140 includes the freeze barrel 48, it also includesfurther structure (not shown) that is generally duplicative of thestructure shown to the right of the brix valves 102, 84, to accommodatedelivery of a water and syrup mixture from the brix valves 104, 87 tothe barrel 48, except that the beverage mixture does not flow through aseparate pre-chiller, but instead flows through an associated beveragecircuit of the pre-chiller 52. In addition, the line 126 at the outletfrom the manifold third CO₂ pressure regulator delivers CO₂ to an upperopening to an expansion chamber (not shown), a lower opening from whichis coupled to the inlet to the barrel 48, and to accommodate carbonatingthe beverage mixture delivered to the barrel 48, an outlet from a secondCO₂ pressure regulator of the manifold 118 is coupled through thesolenoid shut-off valve 128, the CO₂ flow control valve 130 and the CO₂check valve 132 to the chilled beverage mixture intermediate thepre-chiller 52 and the inlet to the barrel 48.

A further type of FCB dispenser with which the refrigeration system 20may be used and operated according to the prescriptive refrigerant flowcontrol scheme of the invention, and that utilizes in-line coldcarbonation, is illustrated in FIG. 5 and indicated generally at 180. Asfor the previously described embodiments, it is understood that onlysomewhat more than one-half of the dispenser is illustrated and that anadditional portion that would include the freeze barrel 48 is not shown,but is part of the dispenser. To deliver a first flavor of syrup to thedispenser, an externally pumped first syrup supply is coupled through aline 182 to an inlet to syrup brix valves 184, with a switch 183detecting exhaustion of the first syrup supply. To deliver a secondflavor of syrup to the dispenser, an externally pumped second syrupsupply is coupled through a line 185 to an inlet to a syrup brix valve186, with a switch 187 detecting exhaustion of the syrup supply. Todeliver water to the dispenser, potable water from a city water main iscoupled through a strainer/regulator 190 to inlets to each of two waterpumps 192 and 194, and a pressure switch 196 is coupled to thestrainer/regulator to detect a water-out condition. Water at outletsfrom the pumps 192 and 194 is delivered through associated fluidcircuits in the pre-chiller 52 for being cooled, with water from thepump 192 then being delivered through a check valve 206 to a waterrefill inlet to a carbonator tank 198, and then from an outlet from thecarbonator tank to an inlet to a water brix valve 210 associated withthe syrup brix valve 186. In turn, water from the pump 194 is deliveredthrough a check valve 208 to a water refill inlet to a carbonator tank202, and then from an outlet from the carbonator tank to an inlet to awater brix valve 212 associated with the syrup brix valve 184. Tocarbonate water in the carbonators 198 and 202, an external supply ofCO₂ is coupled to inlets to three pressure regulators of a manifold 212,and an outlet from a first pressure regulator of the manifold is coupledthrough a solenoid controlled valve 214 to a CO₂ inlet to the carbonator198 and an outlet from a second pressure regulator of the manifold iscoupled through a solenoid controlled valve 215 to a CO₂ inlet to thecarbonator 202, with a sensor 216 being provided to detect a CO₂ outcondition. When carbonation of water in the carbonator tanks 198 and 202is desired for service of carbonated frozen beverages, the solenoidcontrolled valves 214 and 215 are operated to their open states. Whenwater in the carbonator tanks is not to be carbonated, the valve 214 and215 are operated to their closed state. An output from a third pressureregulator of the manifold 212 is applied to an upper inlet to anexpansion chamber 226 as well as through a line 227 to an upper inlet toan expansion chamber associated with the freeze barrel 48 (neithershown).

Each pair of water/syrup brixing valves 210, 186 and 212, 184 isadjustable to provide a selected water/syrup ratio to its associatedfreeze barrel 48 and 44. In this connection, a common outlet from thevalves 212, 184 is coupled through a 3-way valve 218 to an inlet to thefreeze barrel 44. The 3-way valve has an outlet 220 leading to ambient,whereby a water and syrup beverage mixture supplied by the brixingvalves 212, 184 may be collected for analysis. A pressure transducer 299senses the pressure of the beverage mixture delivered to and within theproduct freeze barrel 44 and operates as a cut-in/cut-out sensor in amanner as described in connection with the FCB dispensers of FIGS. 3 and4.

The FCB dispenser 180 includes the freeze barrel 48 (not shown). Acommon outlet from the brixing valves 210, 186 is delivered through anassociated 3-way valve (also not shown) to an inlet to the barrel 48,and a pressure transducer and an expansion tank are coupled to the inletto the barrel (neither shown). Operation of the dispenser 180 inproviding frozen beverage product from the freeze barrels 44 and 48 isunderstood by those skilled in the art in view of the above-describedoperation of the dispensers 80 and 140.

As discussed, the load requirements of an FCB machine are highlyvariable, since customer demand for frozen beverage product dispensedcan vary from no drinks being served to as many as 3 or 4 or more drinksserved per minute. Because the demand for Btu cooling output by therefrigeration system is so variable and rapidly dynamically changing, itis not practical to rely upon a typical control loop using temperaturesensors for feedback to control operation of the refrigeration system.Temperature sensors are too slow, having a 1^(st) order response time onthe order of 5 seconds or more, and cannot provide meaningful outputs ina sufficiently timely manner to enable the refrigeration system to keepup with changes in cooling load demand. While temperature sensors wouldbe responding to changes in temperature, such as occur in a freezebarrel during refill of the barrel, changes in the cooling load that therefrigeration system is required to meet can be occurring at a ratefaster than the temperature sensors can respond to, and therefrigeration system control cannot keep up.

Ideally, as cooling loads imposed on the refrigeration system 20 by anFCB machine rapidly change due to volatile customer demand for frozenbeverages, the speed of operation of the refrigeration system compressorand the metering positions of the expansion valves immediately followthe cooling load and adjust to more appropriate settings. The idea is toclosely match the cooling capacity of the refrigeration system to theimmediate and dynamically changing cooling load requirements.

For each frozen beverage drawn from a freeze barrel, there is a “batch”of cooling that must be provided by the refrigeration system to the FCBdispenser. The batch of cooling can be expressed in terms of the numberof Btu's of cooling that must be provided by the refrigeration system tothe dispenser (i.e., Btu's of heat that must be extracted by therefrigeration system from the dispenser) to chill and properly freezethe relatively warm beverage product that is flowed through thedispenser to the freeze barrel to refill the barrel, which number ofBtu's is dependent upon the volume and ambient temperature of thebeverage product flowed through the dispenser. As multiple frozenbeverages are drawn, the batches of cooling Btu's required to beprovided by the refrigeration system increase. Since the flow rate ofliquid beverage product through a pair of brixing valves can be closelyapproximated and the temperature of beverage product flowed into the FCBdispenser can be sensed, the batches of Btu's required to chill andfreeze the beverage product can be closely correlated with the timeduration of actuation, or on time, of the brixing valves that deliverthe beverage product mixture to a freeze barrel, together with thetemperature of the beverage product. Thus, given the on time of thebrixing valves and the temperature of the beverage product flowed intothe dispenser, the Btu's of cooling required for a batch of beverageproduct passing through the dispenser can readily be determined.

Depending upon the number of drinks drawn, the total number of Btu'srequired to satisfy the cooling load will vary and generally equal thesum of the batches of Btu's as measured by the control system in termsof total brix valve on time and ambient temperature of liquid beveragecomponents delivered to the FCB machine. Because there is variation inthese parameters over time, a 30-second rolling average can be used tosmooth the information, such that the control system that utilizes theprescriptive refrigerant flow control responds primarily to the mostrecent information. For any average value of brixing valve on time, avolume flow of refrigerant required to satisfy the cooling load can becalculated, based upon which the refrigeration system compressor isoperated at a selected speed, and the expansion valves are operated toselected metering positions, such that the required refrigerant volumeflow occurs in a selected cycle time.

In general, as warm beverage product flows into the FCB dispenser withopening of brix valves, the number of Btu's required to chill and freezethe product is calculated and introduced into a Btu_(total) counter ofthe prescriptive control system. If there is a Btu count already storedin the counter, the latest count is added to it. When there is a countin the Btu_(total) counter representing an amount of cooling that mustbe provided by the refrigeration system for the dispenser to freezeproduct in the barrels, a determination is made of the mass flow ofrefrigerant required to be delivered by the variable speed compressorand through the expansion valves to provide a number of cooling Btu'sgenerally equal to the count stored in the Btu_(total) counter, as wellas the cycle time over which the mass flow of refrigerant is to bedelivered. The speed of the compressor and the degree of opening of theexpansion valves are then controlled to deliver the mass flow ofrefrigerant in a controlled manner over the cycle time determined. Asthe mass flow of refrigerant is delivered, the count in the Btu_(total)counter is decremented in accordance with the amount of cooling Btu'sprovided by the refrigeration system. An objective is to calculate thenumber of Btu's required to properly freeze beverage product flowed intothe FCB dispenser, and then operate the refrigeration system in suchmanner as to efficiently provide the necessary cooling to the dispenser.

Consideration will now be given to the fundamental strategies employedin operation of the refrigeration system in accordance with theprescriptive refrigerant flow control scheme of the invention. Anobjective is to manage the flow of liquid refrigerant such thatsub-cooled liquid is maintained upstream of the expansion valves which,as mentioned, may be of the stepper motor driven or pulse valvemodulated type, while minimizing any liquid refrigerant carryover fromthe evaporators to limit the potential for damage to the compressor. Toaccomplish this objective, it is contemplated that when there is a countin the Btu_(total) counter representing a cooling load to be satisfiedby the refrigeration system, the expansion valves be prescriptivelypositioned, i.e., be positioned according to predetermined rules orcriteria, and the compressor be operated at a selected speed, so as tocause the output from the refrigeration system to match the cooling loadassociated with drink demand as represented by the count in theBtu_(total) counter.

As mentioned, the total cooling load demand, i.e., the total Btu'srequired to be extracted from the dispenser freeze barrel by therefrigeration system to satisfy the cooling load demand, can be measuredby monitoring the on time of the brix valves supplying beverage productto the FCB dispenser, together with the temperature of the beverageproduct flowed to the dispenser. For example, the nominal flow ofbeverage product through a properly adjusted pair of solenoid controlledbrix valves supplying water and syrup is on the order of 2.0 oz/sec. Anaverage 16 oz (by volume) drink will deplete some of the barrel contentsand will require that the contents be replenished. The time to replenishthe contents of a barrel is approximately 4 seconds for a 16 oz (interms of volume) drink equivalent. The total Btu load imposed on therefrigeration system by the replacement beverage product is equal to thesum of the sensible load plus the freeze load. Each second the sensibleload will vary with ambient temperature, but for the purpose of theprescriptive refrigerant control scheme, the sensible load is calculatedas follows:

Q _(sensible)=2/16·(T _(ambient)−28)=0.125·(T _(ambient)−28)

For an ambient temperature of 90° F., Q_(sensible)=7.75 Btu

The freeze load is a constant value, and is calculated as follows on aper second basis:

Q _(freeze)=2/16·(0.46·144)=0.125·66.24=8.28 Btu

This means that 16.03 Btu of heat load (Q_(ambient)+Q_(freeze)) is addedto the FCB refrigeration system every second that relatively warmproduct is flowing into the machine as a result of actuation of a pairof brix valves to replenish a freeze barrel. For a 16 oz (by volume)drink equivalent, the total heat added will be approximately 16.03×4seconds, or 64 Btu's, at a 90° F. ambient condition.

The prescriptive control for the FCB machine determines and counts Btu'sof heat flowing into and through the machine and that must be extracted,and as new product flows into the machine, the Btu_(total) counter isincremented by a count in accordance with the number of Btu's of coolingthat must be provided by the refrigeration system to chill and freezethe new product. The Btu_(total) counter is updated every second, andduring times when no new product flows into the FCB dispenser, theBtu_(total) counter is decremented to zero over a selected period oftime, for example over 40 seconds, as Btu's of heat are removed from thedispenser by the refrigeration system. In other words, if no new productflows into the FCB machine, the total number or count of Btu'saccumulated in Btu_(total) counter will decrement to zero in 40 secondsand the cooling requirement will then be ended or close to ending. Oncethe Btu_(total) counter decrements to zero, the final factor thatresults in a determination that product in the barrels has been properlyfrozen and the refrigeration system can be turned off is the viscosityof the frozen product in the FCB barrels, which may measured as afunction of the magnitude of the current draw of motors for scrapers inthe barrels. Under conditions when no drinks are drawn and there is nodemand for frozen beverages, the refrigeration system will be operatedaccording to pre-selected cooling rates, just as it will when there is acooling load requirement to be met.

FIGS. 6-10 illustrate representative operating conditions of the FCBdispenser. The chart of FIG. 6 shows contemplated speeds of operation ofthe refrigeration system compressor 22, as might be determined by theprescriptive control in order to adjust the output capacity of thecompressor in accordance with the count in the Btu_(total) counter. Thecount in the Btu_(total) counter is related to the cooling load demandplaced on the refrigeration system by the dispenser, and in FIG. 6 thecount is in accordance with the average number of frozen beveragesserved per minute. As is seen, based upon the average number of drinksserved per minute, and the average number of actuations and the on timeper minute of the brixing valves, the speed of operation of thecompressor 22 is controlled to provide a variable capacity Btu coolingoutput by the refrigeration system in accordance with whether therefrigeration system is to meet a maintenance load or a low, medium,high or very high cooling demand of the dispenser.

The chart of FIG. 7 shows how compressor speed may be based upon the FCBdispenser being in pull down, freeze product or maintenance mode.

As seen from the chart of FIG. 8, a drink demand rate in excess of 1×16oz drink per minute, as determined by on time per minute of the brixingvalves, may be considered a period of medium to very high cooling loaddemand, requiring refrigeration of the freeze barrels 44 and 48 and thepre-chiller 52. The pre-chiller is refrigerated whenever a pair ofbrixing valves is actuated.

The chart of FIG. 9 shows typical cooling load profiles placed on therefrigeration system by the FCB dispenser at various drink dispenserates and ambient temperatures, where in the legend box to the right ofthe chart “OR” stands for overrun, which is the amount of beverage in acup attributable to carbonation.

The chart of FIG. 10 illustrates typical cooling load requirements forthe beverage product cooling modes of pre-chilling, freezing andmaintenance, at an ambient temperature of 75° F., for various drink drawrates. The chart of FIG. 11 is similar to that of FIG. 10, except thatcooling load requirements are shown for an ambient temperature of 90° F.

The control strategy and function table of FIG. 12 shows contemplatedmanners of operation of the variable speed refrigeration systemcompressor in response to various FCB machine cooling loads, asrepresented by drinks drawn per minute and, therefore, the on time perminute of the brix valves. FIG. 13 provides a graphical representationof various possible speeds of operation of the refrigeration systemcompressor in accordance with the number of frozen beverages served perminute.

In implementing prescriptive refrigerant flow control, the mass flow ofrefrigerant through the expansion valves for the freeze barrelevaporators is controlled, in accordance with the count in theBtu_(total) counter, by adjustably setting the expansion valves to meterselected amounts of liquid refrigerant within determined cycle times inorder to provide selected amounts of cooling for the FCB dispenser. Atthe same time, the speed of the compressor is controlled so as not todeliver liquid refrigerant to the expansion valves at a ratesignificantly greater than the valves can meter the refrigerant. Todevelop a metering setting for the expansion valves, once the volumeflow of relatively warm beverage liquid into the dispenser is inferredfrom a measurement of the on time of the brixing valves and thetemperature of the incoming beverage liquid, and the count in theBtu_(total) counter is incremented accordingly, an estimate is developedof the refrigerant mass flow required to chill and properly freeze thebeverage liquid flowed into the dispenser. The estimate of refrigerantmass flow may be obtained from a look-up table or calculated directlyand, based upon a selected cooling cycle time during which the mass ofrefrigerant is to flow through the expansion valves, the refrigerantflow rate through the valve may be estimated and, thereby, the valvesmay be set to metering positions that will accommodate the estimatedrefrigerant flow rate in the selected cooling cycle time. The initialsettings of the expansion valves can be a “best guess”, and it iscontemplated that either ambient temperature or, alternatively,condensing pressure, may be used as a variable to more preciselyprescribe the initial expansion valve settings. The prescriptivecontroller then fine-tunes the metering settings of the expansionvalves.

The graph of FIG. 14 shows a representative relationship of refrigerantmass flow in pounds mass/hour to expansion valve metering positions interms of steps of an actuating stepper motor, for ambient temperaturesof 75° F., 90° F. and 104° F. The graph of FIG. 15 shows arepresentative relationship of refrigerant flow in pounds mass/hour tobeverage product draw rate, which is inferred from measured brixingvalve on-time, for ambient temperatures of 75° F., 90° F. and 104° F.The graph of FIG. 16 shows a representative relationship of refrigerantmass flow in pounds mass/hour to brixing valve on-time, for ambienttemperatures of 75° F., 90° F. and 104° F.

One contemplated control strategy for operating the refrigeration system20 to efficiently responds to dynamically changing broad ranges ofcooling load requirements of an FCB dispenser will now be considered ingeneral terms in connection with the FCB dispenser 80 of FIG. 3, itbeing understood that similar control strategies would apply to use ofthe refrigeration system with other FCB dispensers, or for that matterin other and differing situations where dynamically changing coolingload requirements need to be quickly and efficiently satisfied. If thereis a demand for cooling from a freeze barrel and the refrigerationsystem compressor 22 is off at the time, then based upon the countincremented into the Btu_(total) counter, the compressor is turned onand operated at a selected speed, and refrigerant is metered through theexpansion valve(s) for the freeze barrel(s) at a mass flow rate,commensurate with the cooling load requirement. If at the time of ademand for cooling the compressor is already running, refrigerant ismetered through the expansion valves, and the compressor is operated ata speed, in accordance with the then existent cooling load requirementsof the dispenser, as indicated by the count in the Btu_(total) counter.If the brix valves have not been actuated for a period of time, it isassumed that beverage product flow rates through the dispenser and,therefore, beverage product cooling load requirements, are low, and thatonly a maintenance cooling load need be satisfied, under which conditionthe compressor 22 is brought to a low running speed equal to about 50%of its nominal speed, by application of a 30 Hz AC voltage to thecompressor motor.

To develop an indication of customer demand for frozen beverages and,therefore, the cooling load demand of the FCB dispenser, the time ofactuation of the brix valves and the ambient temperature of the beverageliquid as flowed into the dispenser are monitored to determine thecooling load to be satisfied. For each drink drawn, the “batch of Btu's”that must be provided by the refrigeration system to chill and properlyfreeze replenishment beverage product is incremented into theBtu_(total) counter, and as multiple drinks are drawn, the batchesmultiply and the count in the Btu_(total) counter increases. The flowrate of water and syrup through the brixing valves can be closelyapproximated, so the amount of replenishment beverage product deliveredby the brixing valves to the freeze barrels can be correlated with theon-time of the brixing valves, which in turn relates to the cooling loadthat must be met by the refrigeration system. The cooling load, in termsof Btu's required to chill and freeze each batch of beverage productflowed from the brixing valves, can be calculated and is based upon twofactors: 1) the size of the batch, which is directly related to on-timeof the brixing valves, and 2) the ambient temperature of the beveragecomponents delivered by the brixing valves. As additional beverageproduct is delivered by the brixing valves, the Btu_(total) counter isupdated and incremented on a second by second basis, while at the sametime, as the refrigeration system extracts heat from the FCB dispenser,the count in the Btu_(total) counter is decremented in accordance withthe cooling Btu's supplied by the refrigeration system. During timeswhen frozen drinks are not being dispensed and no new beverage productflows from the brixing valves, the refrigeration system continues to runas necessary to extract heat from the dispenser and to decrement thecount in the Btu_(total) counter to zero over a selected period of timethat may be, for example, on the order of 40 seconds. Consequently, ifno new product flows from the brixing valves for the selected time, thecount accumulated in the Btu_(total) counter will be decremented to zeroand the cooling requirement of the refrigeration system will be close toending. However, while the count in the Btu_(total) counter is intendedto represent the total cooling load requirements of the FCB dispenser,some inaccuracies can exist, and decrementing the Btu_(total) counter tozero is therefore not determinative to turning off the refrigerationsystem. The final factor that shuts off the refrigeration system is themeasured viscosity of the frozen beverage product, which may be measuredas a function of the sensed current draw of motors for the freeze barrelscrapers.

Since the count in the Btu_(total) counter is indicative of theimmediate cooling load demand placed on the refrigeration system 20 bythe FCB dispenser 80, should the count be increasing and indicate thatcooling load requirements are increasing faster than they are being met,then an increase in compressor speed and/or refrigerant metering rate ofthe expansion valves is required to increase the Btu output capacity ofthe refrigeration system. In this case, the speed of the compressor canbe initially incremented by 10% of its present speed, such that shouldthe compressor be operating at 50% nominal speed, then the frequency ofthe AC voltage applied to the compressor motor would be increased by 10%to increment compressor speed to 55% nominal speed. However, duringpull-down, as will be described below and as occurs when the FCBdispenser is initially turned on, the increment in compressor speed willbe more aggressive, for example on the order of 50% to 60% every 5seconds.

Pull-down mode occurs when the FCB dispenser 80 is first turned on afterbeing off, such that the freeze barrels 44 and 48 are warm. Under thiscircumstance, the refrigeration system 20 is controlled to quickly dropthe temperatures of the freeze barrels, the objective being to rapidlybring product in the barrels to within predetermined temperature andviscosity ranges, so that warm drinks are not dispensed. Producttemperature may be determined by temperature sensors and productviscosity is related to, and may be determined in accordance with, ameasurement of current draw in amperes of each motor that rotates ascraper in an associated one of the barrels. In pull-down mode, thecompressor 22 is turned on and the expansion valves 36 and 38 arecontrolled to meter refrigerant to the evaporators of the freezebarrels. When the compressor is first turned on, it is contemplated thatit be run at about 50% nominal speed and then be ramped up in speed from50% nominal speed to 150% nominal speed over a selected period of time,for example over 25 seconds, in which case compressor speed would beincreased in increments of about 10% every 5 seconds. Product is not tobe dispensed from a freeze barrel if its temperature is above and itsviscosity is below predetermined ranges or specifications, so a lock forthe dispense valve 82 can be provided to prevent dispensing of productfrom the valve when beverage temperature is above or beverage viscosityis below specification, or when the barrels are being defrosted.

As the freeze barrels 44 and 48 are cooled, beverage product within thebarrels will be brought to a desired temperature range, generallybetween about 24° F. to 28° F., and the viscosity of the product, asdetermined by sensed scraper motor current draw, will be brought tobetween a selected Lo Limit Value and Hi Limit Value. Once product inthe barrels is brought to within the selected temperature and viscosityranges, the compressor is turned off until further refrigeration isrequired. The schedule for compressor speed operation advantageously isbased upon demand for drinks dispensed, as represented by the on-time ofthe brixing valves, since it is the relatively warm beverage mixturedelivered into the FBC machine that must be chilled and that places acooling load on the refrigeration system 20. When no frozen beveragesare being dispensed, barrel maintenance occurs, during which periodsbarrel refrigeration may be initiated if product viscosity drops to theLo Limit Value or product temperature increases to at least the upperend of the selected temperature range. To reduce beverage producttemperature before delivery of the product to a freeze barrel, whenevera pair of brix valves 102, 84 and 104, 87 is actuated to deliverbeverage product mixture to a freeze barrel, the pre-chiller expansionvalve 40 is operated to cool the pre-chiller 52. It is contemplated thatpre-chilling begin as soon as there is a call for the brix valves toopen, since refrigeration of just the freeze barrels may not besufficient to meet cooling loads that are both high and sustained.

The product freeze barrels 44 and 48 are automatically refilled basedupon internal barrel pressure. For example, when the cut in/cut outpressure sensor 124 detects pressure within the freeze barrel 44decreasing to about a 20 psi cut-in pressure, the brix valves 102, 84are opened to provide beverage product through the pre-chiller 52 to thebarrel, and a similar operation occurs in refilling the freeze barrel48. During refilling of a freeze barrel, the electronic expansion valve40 is opened to meter refrigerant to the evaporator 50 to cool thepre-chiller, so that the beverage mixture is chilled before beingdelivered into the barrel. The brix valves then remain open untilinternal freeze barrel pressure reaches about a 28 psi cut-out pressure,whereupon the brix valves are closed. If necessary, all threeevaporators 42, 46 and 50 can be cooled simultaneously to facilitatepre-chilling and freezing of product in both barrels simultaneously,since the compressor 22 is selected to have sufficient capacity tohandle such a maximum cooling load. In order that a non-flowing beveragemixture within the pre-chiller will not be frozen, the pre-chiller isnot cooled by itself in the absence of a need to cool at least one ofthe product freeze barrels 44 and 48. Upon the brixing valves closingand the temperature of the beverage product in the pre-chiller droppingto about 36° F., the expansion valve 40 for the pre-chiller is closed,although continued cooling of beverage product in the pre-chiller willcontinue for a limited time due to thermal storage capacity of thepre-chiller. Upon the temperature and viscosity of beverage product ineach freeze barrel 44 and 48 being brought to within the selectedtemperature and viscosity ranges, the compressor 22 is turned off.

If when the compressor 22 is running at a relatively high outputcapacity there is little or no heat load imposed by the product barrels44 and 48, or if demand for product suddenly stops, there will veryquickly be excess and unutilized compressor cooling capacity. If thecompressor were to continue running in that mode, the expansion valves36 and 38 would close down and suction pressure at the outlets from theevaporators 42 and 46 would drop to a very low value. The compressorwould then pull down the temperature of product in the barrels and wouldhave to be shut off to prevent excessive freezing of product. Toalleviate this potential problem, the capacity of the refrigerationsystem 20 is varied by varying the speed of the compressor, such that ascooling load demand drops, as may be measured by a reduction in thecount in the Btu_(total) counter, compressor speed is reduced in 5%increments, until 50% nominal speed is achieved. Advantageously,compressor speed should be reduced to 50% nominal speed before barrelproduct temperature and viscosity conditions are fully satisfied, orbefore compressor suction pressure (or saturated evaporator temperature)drops to a selected lower limit. Since cooling load demand isconveniently defined in terms the brixing valves 102, 84 and 104, 87being actuated or opened, cooling loads may be considered to be high ifthe brixing valves are actuated more than 2 times per minute, and may beconsidered low if the brixing valves are actuated less than 2 times perminute. It is contemplated that if actuation of a pair of brixing valvesis less frequent than 1×16 oz drink per minute, the compressor can beoperated at 50% speed. When compressor speed is reduced andrefrigeration cooling capacity is reduced, a saturated evaporatortemperature of about 4° F. will continue to cool product in a barrel,until both temperature and viscosity conditions of product in the barrelare satisfied, whereupon the compressor shuts off and the speed of thebeater bar or scraper in the barrel may be reduced to half speed.

There are four modes of operation for the FCB dispenser 80 when therefrigeration system is active, as seen in FIG. 17. A first mode is forpull-down, which occurs when the FCB dispenser is warm and first turnedon, at which time it is desirable to quickly bring product in the freezebarrels to its desired temperature and viscosity ranges. Duringpull-down the refrigeration system is maintained at a first fixedcooling output in which a fixed expansion valve position is modified bythe cooling rate and system differential pressure. A second mode is formaintenance, which occurs when the dispenser is idle and drinks are notbeing drawn and during which the refrigeration system is maintained at aselected second fixed cooling output in which a fixed expansion valveposition is modified by the cooling rate and system differentialpressure. A third mode is glide mode, which occurs when product in abarrel is approaching, but has not quite reached, its maximum viscosity.It is desirable to prevent the viscosity of the product fromovershooting its maximum value, so in glide mode cooling is continued,but reduced below that for maintenance mode, and the refrigerationsystem is operated at a third fixed cooling output in which a fixedexpansion valve position is selected as a percentage of what would bethe maintenance mode position. A fourth mode is a demand for drinksmode, in which the refrigeration system is operated to have a variablecooling rate and in which the expansion valves have a modified positionbased upon both cooling rate, which is updated every second, andrefrigeration system differential pressure.

FIG. 18 illustrates a contemplated prescriptive control strategy forsetting the metering position of the refrigeration system expansionvalves 36 and 38, to provide selected refrigerant flows through thevalves to the freeze barrel evaporators 42 and 46. To implement thestrategy, at a box 400 the average on time of a pair of brix valves, andat a box 402 the temperature of beverage product flowed through the FCBdispenser in response to actuation of the brix valves, are measured andinput to a box 404. At the box 404, the inputs are used to determine thetotal Btu output Q that is required to be provided by the refrigerationsystem to chill and properly freeze the beverage product flowed throughthe brix valves, and a determination is also made of a cycle time CT inwhich the cooling demand is to be satisfied. The value of CT can bedetermined from a look-up table based upon the value of the count in theBtu_(total) counter, and at a box 406 the values of Q and CT, along withthe value of either ambient temperature or refrigeration system highside pressure as are detected at a box 408, are used to calculate thechange in enthalpy ΔH that is required to fully vaporize a mass ofrefrigeration liquid as will be required to properly freeze the beverageproduct flowed from the brix valves. At a box 410, the mass flow ofrefrigerant M_(dot) in pounds per hour is determined in accordance withthe values of Q, CT and ΔH, and based upon the value of M_(dot), at abox 412 a determination is made, from a look-up table, as to the numberof steps to drive a stepper motor if the expansion valve is steppermotor driven, or as to a pulse width to be used if the expansion valveis of a pulse valve modulated type, in order to position the expansionvalve to meter the determined mass flow of refrigerant M_(dot) in theselected cycle time CT. At a box 414, a determination is made as towhether to increment the Btu_(total) counter if additional drinks aredrawn, or to decrement the counter if no further drinks have been drawn,by a count in accordance with the value of Q i.e., the number of Btu'sthat are to be provided by the refrigeration system. Depending uponwhich expansion valve 36 or 38 is to be controlled, at a box 416 acommand is generated to position the expansion valve 36, or at a box 418a command is generated to position the expansion valve 38, according tothe number of stepper motor steps or pulse width as determined at thebox 412. The command is implemented at a box 420, which applies acontrol signal to an expansion valve driver, such as a driver 422 forthe expansion valve 36.

As seen in FIGS. 19A and 19B, a microprocessor controls operation of theFCB dispenser and its refrigeration system. The microprocessorimplements the prescriptive control and includes the Btu_(total)counter. In implementing the prescriptive control, the microprocessorreceives inputs from the FCB dispenser in accordance with controlstrategies that are to be implemented and that enable a determination asto the state of the cooling demand. For example, since barrelrefrigeration affects product temperature and product viscosity, andsince as product temperature goes down product viscosity goes up, it iscontemplated that the cooling capacity of the refrigeration system 20 bedecreased by reducing the refrigerant flow rate and drying out theevaporator coil in accordance with the count in the Btu_(total) counter.Also, based upon a pressure differential between compressor suction anddischarge, it is contemplated that the expansion valves bepre-positioned to a selected setting, which setting can be in accordancewith the count in the Btu_(total) counter. In addition, it iscontemplated that as cooling load increases or decreases, as determinedby the change in the count in the Btu_(total) counter, the speed of thecompressor be modulated proportionally to the change in cooling load.Also, once the cooling load drops below the modulating speed range ofthe compressor, continued modulation of the cooling capacity of therefrigeration system 20 may be accomplished by modulating the positionof the expansion valves in accordance with the count in the Btu_(total)counter, as determined by brix valve actuation.

Before considering the FIG. 20 flow chart depicting operation of therefrigeration system and frozen product dispenser cooled by therefrigeration system, certain relationships between cooling loads,compressor speeds and expansion valve metering positions will bediscussed in general terms. Ideally, the expansion valve meteringposition is determined by matching the liquid refrigerant flow ratethrough the expansion valve and the compressor pumping rate. As thecooling load placed on the evaporator by the dispenser drops off, thecompressor will have excess capacity, and the capacity of the compressormust be reduced using modulation techniques, such as by reducing thecompressor speed. Conversely, if the cooling requirement increases, asmeasured by the amount of liquid product flowing through the dispenserto a freeze barrel, the compressor speed must be increased. Ascompressor speed is modulated, the expansion valve metering position iscontrolled to be in accordance with compressor pumping rate, and theintention of the prescriptive refrigerant flow control is for theexpansion valve metering position to closely, but not necessarilyperfectly, match the compressor pumping rate. However, the expansionvalve metering position, while it can be directly responsive tocompressor speed, need not be directly responsive. The variable speedcompressor can be prescriptively controlled to respond directly tocooling load changes, as measured by brix valve on time. At the sametime, the expansion valve metering position can also be prescriptivelycontrolled to respond directly to brix valve on time. Therefore, each ofcompressor speed and expansion valve metering position can be separatelyand independently prescriptively controlled as a function of brix valveon time, and it is not necessary that there be a direct dependency ofone on the other, but rather each can be made to independently trackcooling load as measured by brix valve on time.

Advantages are obtained by having the prescriptive expansion valvemetering settings be responsive to cooling load changes and notcompressor speed changes. The range over which the compressor is able torespond to changing cooling loads is limited and does not necessarilyencompass the entirety of the cooling load range. Once the limits ofcompressor speed modulation are reached, then the expansion valves mustcontinue to open or close, based upon increases or decreases in coolingload requirements, and at that point the dependency expansion valvemetering position on compressor speed would no longer be valid. At lowcooling loads, where minimum compressor capacity has been reached, theexpansion valves must further restrict refrigerant flow to avoid liquidcarryover from the evaporators. On the other hand, at high loads wheremaximum compressor capacity has been reached and is less than evaporatorcooling capacity, the expansion valve must open up to increase theevaporator cooling capacity. In the case of a fixed speed compressor,the need to track cooling load becomes most apparent, while thedependency of the expansion valve metering position on compressor speedbecomes non-existent.

The flow chart of FIG. 20 illustrates operation of the refrigerationsystem in providing cooling for an FCB dispenser in accordance with theprescriptive refrigerant flow control scheme of the invention, as may beimplemented by the FIG. 20A CPU. The chilling of each freeze barrel ofthe dispenser is separately prescriptively controlled, such that controlover chilling of each freeze barrel would be represented by a separateflow chart. The flow chart therefore applies to just one freeze barreland its associated evaporator, expansion valve and brix valves, and itwill therefore be described in connection with providing cooling forjust a single freeze barrel of the dispenser. However, it is to beappreciated that a similar but separate control is implemented by theCPU for each additional freeze barrel and its associated evaporator,expansion valve and brix valves, and that while chilling of each freezebarrel is separately controlled, all of the freeze barrels of thedispenser share the refrigeration system compressor 22 as well as thevarious sensors as are indicated in FIG. 20B.

The prescriptive refrigerant flow control scheme is performedcyclically, on the order of once per second. Beginning at a start 500,at a box 502 it is determined whether a product call exists, i.e.,whether the brix valves have been actuated to deliver relatively warmbeverage product to a freeze barrel of an FCB dispenser. If the brixvalves have been actuated, at a box 504 it is determined whether a firstdrink flag is set, which first drink flag is set in response to the brixvalves being closed at a time while the compressor 22 is off. If a firstdrink flag has been set, it is cleared at a box 506 and at a box 508 thecount BTU T in the Btu_(total) counter is made equal to:

BTUT=BTUT+f(PA,BinT)+ΔBTU

where:f(PA,BinT)=(PA·PFR/16)·[(BinT−28)·Cp+144·0.46]

and: PA is product call or brix valve actuation time in seconds;

-   -   PFR is the product mass flow rate and equals 2 ounces        mass/second;    -   16 is for a conversion to pounds mass/second;    -   28 is the temperature in ° F. at which the product will freeze;    -   BinT is the temperature of product delivered into the freeze        barrel in ° F.;    -   Cp is the specific heat of water;    -   144 is the Btu/pound mass required to freeze water;    -   0.46 is the ice fraction for an FCB drink; and    -   ΔBTU is the cumulation of BTUs in the BTU_(total) counter as a        result of additional beverages being drawn prior to the BTU        demand resulting from the first beverage drawn being satisfied.        At a box 510, ΔBTU is set to zero, and at a box 512 the cycle        time CT is read, which cycle time CT can be selected and        changed, but once selected normally remains fixed. The cycle        time CT may be on the order of about 40 seconds, and is the time        period in which the refrigeration system 20 is operated to        provide to the freeze barrel an amount of cooling equivalent to        the number of Btu's indicated by the count BUT T in the        Btu_(total) counter, thereby to freeze the relatively warm        beverage product delivered by the brix valves to the freeze        barrel. At a box 514, the cooling rate CR of the freeze barrel        in Btu's/second, which is the rate at which the freeze barrel is        to be chilled by the refrigeration system in order to freeze        product in the barrel within the cycle time CT, is determined        according to the formula:

CR=BTUT/CT

where CR is expressed in Btu/second and is the instantaneous coolingrate of the freeze barrel evaporator. The count BTU T in the Btu_(total)counter is decremented as the freeze barrel is chilled, but it is notthe value of BTU T that determines when sufficient cooling has beenprovided for the freeze barrel. Instead, the viscosity of beverageproduct in the freeze barrel, as represented by the current draw of amotor for a scraper in the barrel, is determinative as to whensufficient cooling has been provided by the refrigeration system, and ata box 516 a determination is made whether:

V>Max V−ΔV

where V is the product viscosity, Max V is the maximum viscosity theproduct is to be brought to, and ΔV is a predetermined viscosity valuefor the frozen beverage at which point the glide rate of refrigerantflow control is to be implemented. If product viscosity is greater thanMax V−ΔV, at a box 518 the cooling rate CR of the freeze barrelevaporator is changed to a glide rate, which is a reduced andpredetermined rate of cooling that is used when product viscosity isclose to its maximum viscosity Max V, so that the product is brought toMax V relatively slowly and with minimal overshoot. From either the box516 or the box 518, the control goes to a box 520 where it is determinedif product viscosity V is greater than Max V, and if it is thatindicates that product in the freeze barrel has been sufficiently cooledand at a box 522 the refrigeration system 20 is controlled so that thecooling rate CR of the freeze barrel evaporator is brought to zero,which is accomplished by closing the expansion valve for the evaporator.From the box 522, as well as from the box 520 if a determination is madethat product viscosity V is less than Max V, the control goes to a box524 where the mass flow MF of refrigerant to be provided by therefrigeration system compressor 22 and metered by the expansion valvefor the freeze barrel evaporator is determined according to the formula:

MF=f(CR,PSIH)

where:f(CR,PSIH)=CR/(91.47−(0.1125·PSIH+13.696))

and: 91.47 is the estimated enthalpy value for refrigeration system lowside at 10° F.;

PSI H is the refrigeration system high side pressure; and

0.1125·PSI H+13.696 is the enthalpy value for refrigeration system highside.

After determining MF, at a box 526 the value of PW, which can representeither the number of steps to drive a stepper motor or the pulse widthto be used to control the metering rate of the expansion valve,depending on whether the expansion valve is stepper motor or pulse widthdriven, is determined according to:

PW+f(MF,OPD)

where OPD is the refrigeration system operating pressure differential(PSI H−PSI L). Having determined PW, the metering rate of the expansionvalve is adjusted accordingly, such that the instantaneous cooling rateof the expansion valve is brought to the determined cooling rate CR, andat a box 528 the prescriptive refrigerant flow control returns to start500.

If, after beginning at the start box 500, at the box 502 it isdetermined that there is no product call activated such that the brixvalves are closed, at a box 530 it is determined whether the count BTU Tin the Btu_(total) counter is less than or equal to zero. If it is notand BTU T is greater than zero, at a box 532 BTU T is reset accordingto:

BTUT=BTUT−f(CMFR)

where CMFR is compressor refrigerant mass flow rate and may either becalculated according to f(PSI H−PSI L) or determined through use ofeither a look-up table for the compressor or a look-up table for theexpansion valve. At a box 534, the remaining cycle time CT in which therefrigeration system is to provide the required cooling to theevaporator of the freeze barrel is decremented by one second, and at abox 536 a cooling flag for a standard 16 ounce beverage is cleared.Then, at the box 514 the cooling rate CR is determined, following whichthe prescriptive refrigerant control proceeds as above described.

If, after beginning at the start box 500 and determining at the box 502that the brix valves are actuated, it is determined at the box 504 thatthe first drink flag has not been set, the control proceeds to a box538. If the first drink flag is not set, which is the case on the secondpass through the flow chart following actuation of the brix valves whilethe refrigeration system is providing cooling to the freeze barrelevaporator, or if the brix valves are again actuated to deliver beverageproduct to the freeze barrel while the refrigeration system is chillingthe freeze barrel evaporator, then at the box 538 it is determined if a16 ounce drink flag is set. The 16 ounce drink flag is set when the brixvalves are actuated to deliver product to the freeze barrel incident toservice of a second, third, fourth, etc. drink, while the refrigerationis still operating to satisfy freeze barrel cooling requirements. If the16 ounce drink flag is set, then the cycle moves to the box 508, fromwhich it proceeds as above described. On the other hand, if the 16 ouncedrink flag is not set, then at a box 540 ΔBTU, which is the change incooling load placed on the refrigeration system as represented by thecount BTU T in the Btu_(total) counter, is updated according to:

ΔBTU=ΔBTU+f(PA,BinT)

where again PA is brix valve actuation or opening time in seconds andBin T is the temperature of product delivered to the freeze barrel inletin ° F. If at a box 542 it is determined that the change in coolingrequirements, ΔBTU, is equal to that required to freeze a 16 ouncebeverage, at a box 544 a 16 ounce drink flag is set and the cycleadvances to the box 512. On the other hand, if at the box 542 it isdetermined that the change in cooling requirements ΔBTU does not equalthat required to freeze a 16 ounce drink, then the cycle advancesdirectly to the box 512. From the box 512, the cycle proceeds as abovedescribed.

If at the box 502 it is determined that brix valves are not actuated andif at the block 530 BTU T is less than or equal to zero, then it isassumed that the refrigeration system has satisfied its freeze barrelcooling requirements and at a box 546 a first drink flag is set and at abox 548 both the count BTU T in the Btu_(total) counter and the cycletime CT are set to zero. A determination is then made, at a box 550,whether the freeze barrel is in pull-down mode. If it is in pull-downmode, then at a box 552 the cooling rate CR is set to or maintained at apredetermined increased cooling rate to rapidly chill the freeze barrel,and the control then proceeds from the box 516 in the manner abovedescribed. On the other hand, if at the box 550 it is determined thatthe barrel is not in pull-down mode, at a box 554 it is determined ifproduct viscosity V is less than a predetermined minimum viscosity. Ifthe viscosity is greater than the predetermined minimum, the controlproceeds from the box 516 in the manner above described. On the otherhand, if product viscosity is less than the predetermined minimum, at abox 556 the cooling rate CR is set to a pre-selected maintenance level,whereupon at the box 516 the control proceeds as above described.

While embodiments of the invention have been described in detail,various modifications and other embodiments thereof may be devised byone skilled in the art without departing from the spirit and scope ofthe invention, as defined by the appended claims.

1-28. (canceled)
 29. A refrigeration system, comprising: an evaporatorheat exchange coupled to a load to be chilled to within a selectedtemperature range; an adjustable expansion valve for meteringrefrigerant to said evaporator; a variable speed compressor fordelivering refrigerant to said expansion valve; means for sensing achilling requirement placed on said refrigeration system by the load,that is to be provided by said refrigeration system in order for saidrefrigeration system to cool the load to within the selected temperaturerange; means responsive to said sensing means for selecting the durationof a time interval during and throughout which said refrigeration systemis to provide the sensed chilling requirement to the load; and meansresponsive to each of said sensing and selecting means for operatingsaid refrigeration system to control the speed of said compressor todeliver refrigerant to said expansion valve, and to adjust saidexpansion valve to meter refrigerant to said evaporator, at flow ratessuch that said refrigeration system provides the sensed chillingrequirement to the load during and throughout the selected timeinterval, whereby said refrigeration system provides a cooling outputover a selected time interval that closely matches the chillingrequirement of the load.
 30. A refrigeration system as in claim 29,wherein said means for operating comprises: means responsive to saidsensing means for determining a mass flow of refrigerant through saidexpansion valve to said evaporator that is required to chill the load towithin the selected temperature range and for adjusting said expansionvalve to meter the determined mass flow of refrigerant during theselected time interval; and means for operating said compressor at aspeed to deliver the determined mass flow of refrigerant to saidexpansion valve during the selected time interval.
 31. A refrigerationsystem as in claim 29, wherein the load is a frozen product dispenserand said evaporator is heat exchange coupled to a freeze barrel of thefrozen product dispenser to chill product in the freeze barrel to withinthe selected temperature range, the freeze barrel being of a type fromwhich frozen product is dispensed and into which relatively warm productis flowed to replace dispensed product, whereby the freeze barrel placeson said refrigeration system a chilling requirement that varies inaccordance with the quantity and temperature of the flows of relativelywarm replacement product into the freeze barrel, so that said operatingmeans operates said refrigeration system to control the speed ofoperation of said compressor to deliver refrigerant to said expansionvalve, and to adjust said expansion valve to meter refrigerant to saidevaporator, at flow rates such that said refrigeration system providesthe sensed chilling requirement to the freeze barrel during andthroughout the selected time interval.
 32. A refrigeration system as inclaim 31, wherein said sensing means comprises means for monitoring thequantity and temperature of relatively warm replacement product flowedinto the freeze barrel, and means responsive to said monitoring meansfor determining the number of Btu's of cooling required to be providedby said refrigeration system to the freeze barrel to chill product inthe freeze barrel to within the selected temperature range, saidoperating means being responsive to said sensing means and saidselecting means to operate said refrigeration system to have a coolingcapacity closely matching the immediate and dynamically changingchilling requirements of the freeze barrel.
 33. A refrigeration systemas in claim 32, said sensing means including a Btu counter and meansresponsive to said determining means upon each flow of relatively warmproduct into the freeze barrel for incrementing into said Btu counter acount in accordance with the determined number of Btu's of coolingrequired to be provided by said refrigeration system to chill theproduct then flowed into the freeze barrel to within the selectedtemperature range, and means for decrementing the count in said Btucounter by the number of Btu's of cooling provided by said refrigerationsystem to product in the freeze barrel, whereby the instantaneous countin said Btu counter is representative of the number of Btu's of coolingthen required to be provided by said refrigeration system to chillproduct in the freeze barrel to within the selected temperature range.34. A refrigeration system as in claim 31, including means for detectingthe viscosity of product in the freeze barrel, and means for continuingoperation of said refrigeration system to chill product in the freezebarrel if at the end of the selected time interval the detectedviscosity is less than a predetermined value and for terminatingoperation of said refrigeration system if the detected viscosity is atleast equal to the predetermined value.
 35. A refrigeration system as inclaim 31, wherein said operating means increases the cooling capacity ofsaid refrigeration system in response to a sensed increase in thechilling requirement of product in the freeze barrel while saidrefrigeration system is chilling product in the freeze barrel.
 36. Arefrigeration system as in claim 31, wherein the frozen productdispenser has a product pre-chiller through which replacement productflows for being chilled before flowing into the freeze barrel, saidrefrigeration system further including a second evaporator heat transfercoupled to the product pre-chiller and a second expansion valve formetering refrigerant to said second evaporator, said variable speedcompressor delivering refrigerant to said second expansion valve forbeing metered to said second evaporator to chill product flowed throughthe pre-chiller, said means for operating being responsive to a flow ofproduct through the pre-chiller to the freeze barrel to operate saidcompressor to deliver refrigerant to said second expansion valve and tooperate said second expansion valve to meter refrigerant to said secondevaporator to chill product flowed through the pre-chiller.
 37. A methodof operating a refrigeration system, comprising the steps of: heatexchange coupling an evaporator to a load to be chilled to within aselected temperature range; metering refrigerant through an adjustableexpansion valve to the evaporator; delivering refrigerant to theexpansion valve with variable speed compressor; sensing a chillingrequirement placed on the refrigeration system by the load, that is tobe provided by the refrigeration system in order for the refrigerationsystem to cool the load to within the selected temperature range;selecting, in response to performance of said sensing step, the durationof a time interval during and throughout which the refrigeration systemis to provide the sensed chilling requirement; and operating therefrigeration system, in response to performance of each of said sensingand selecting steps, to control the speed of the compressor to deliverrefrigerant to the expansion valve, and to adjust the expansion valve tometer refrigerant to the evaporator, at flow rates such that therefrigeration system provides the sensed chilling requirement to theload during and throughout the selected time interval.
 38. A method asin claim 37, wherein said sensing step comprises the steps ofdetermining a mass flow of refrigerant through the expansion valve tothe evaporator required to closely match the chilling requirement placedon the refrigeration system by the load in order to chill the load towithin the selected temperature range, and adjusting the expansion valveto meter the determined mass flow of refrigerant during the selectedtime interval, and said operating step operates the compressor at aspeed to deliver the determined mass flow of refrigerant to theexpansion valve during the selected time interval.
 39. A method ofoperating a refrigeration system for a frozen product dispenser having afreeze barrel from which frozen product is periodically dispensed andinto which relatively warm product is flowed to replace dispensedproduct, whereby the freeze barrel places on the refrigeration system aheat load that varies in accordance with the quantity and temperature ofrelatively warm replacement product periodically flowed into the freezebarrel, said method comprising the steps of: heat exchange coupling anevaporator to the freeze barrel to chill product in the freeze barrel towithin a selected temperature range; metering refrigerant through anadjustable expansion valve to the evaporator; delivering refrigerant tothe expansion valve with variable speed compressor; sensing a chillingrequirement placed on the refrigeration system by a flow of relativelywarm product into the freeze barrel, that is to be provided by therefrigeration system in order for the refrigeration system to cool theproduct to within the selected temperature range; selecting, in responseto performance of said sensing step, the duration of a time intervalduring and throughout which the refrigeration system is to provide thesensed chilling requirement; and operating the refrigeration system, inresponse to performance of said sensing and selecting steps, to controlthe speed of operation of the compressor to deliver refrigerant to theexpansion valve, and to adjust the expansion valve to meter refrigerantto the evaporator, at flow rates such that the refrigeration systemprovides the sensed chilling requirement during and throughout theselected time interval.
 40. A method as in claim 39, wherein saidsensing step comprises the steps of monitoring the quantity andtemperature of relatively warm replacement product flowed into thefreeze barrel and, in response to said monitoring step, determining thenumber of Btu's of cooling required to be provided by the refrigerationsystem to chill the relatively warm replacement product flowed into thefreeze barrel, and thereby the product in the freeze barrel, to withinthe selected temperature range during the selected time interval.
 41. Amethod as in claim 39, including the step, responsive to said sensingstep, of determining the mass flow of liquid refrigerant required to bemetered to the evaporator in order for the refrigeration system toclosely match the sensed chilling requirement to thereby chill productin the freeze barrel to within the selected temperature range, saidmetering and delivering steps providing the mass flow of liquidrefrigerant to the evaporator during the selected time interval.
 42. Amethod as in claim 39, said sensing step comprising the steps ofincrementing into a Btu counter, in response to a flow of relativelywarm product into the freeze barrel, a count representative of thenumber of Btu's of chilling required to be provided by the refrigerationsystem to the freeze barrel in order to chill the product flowed intothe freeze barrel, and thereby the product in the freeze barrel, towithin the selected temperature range, and decrementing the count in theBtu counter, in response to chilling of product in the freeze barrel bythe refrigeration system, by the number of Btu's of chilling provided bythe refrigeration system to the freeze barrel, whereby the instantaneouscount in the Btu counter is representative of the number of Btu's ofcooling yet required to be provided by the refrigeration system to chillproduct in the freeze barrel to within the selected temperature range.43. A method as in claim 42, wherein said operating step adjusts themetering rate of the expansion valve and controls the operating speed ofthe compressor so that the refrigeration system has a cooling output,during the selected time interval, that closely matches the chillingrequirement of the freeze barrel and that varies directly in accordancewith the magnitude of the count in the Btu counter.
 44. A method as inclaim 42, wherein said sensing step is performed such that, in theabsence of a flow of relatively warm product into the freeze barrel, thecount in the Btu counter will be decremented to zero by the end of theselected time interval.
 45. A method as in claim 39, including the stepsof monitoring the viscosity of product in the freeze barrel, continuingoperation of the refrigeration system to chill product in the freezebarrel if at the end of the selected time interval the viscosity is lessthan a predetermined value, and terminating operation of therefrigeration system to chill product in the barrel at the end of theselected time interval if the viscosity is at least equal to thepredetermined value.
 46. A method as in claim 39, wherein said operatingstep increases the cooling capacity of the refrigeration system inresponse to a sensed increase in the chilling requirement placed on therefrigeration system by the freeze barrel while the refrigeration systemis chilling the freeze barrel.
 47. A method as in claim 39, wherein thefrozen product dispenser has a product pre-chiller through whichrelatively warm product flows for being chilled before flowing into thefreeze barrel, the refrigeration system further including a secondevaporator heat transfer coupled to the pre-chiller and a secondexpansion valve for metering refrigerant to the second evaporator, thevariable speed compressor also delivering refrigerant to the secondexpansion valve for being metered to the second evaporator to chill thepre-chiller and thereby to chill product flowed through the pre-chillerinto the freeze barrel, said operating step being responsive to a flowof product through the pre-chiller to operate the compressor to deliverrefrigerant to the second expansion valve and to adjust the secondexpansion valve to meter refrigerant to the second evaporator to chillthe second evaporator and thereby to the pre-chiller and product flowingthrough the pre-chiller.
 48. A method as in claim 39, wherein saidsensing step comprises detecting the quantity and temperature ofrelatively warm product flowed into the freeze barrel, determining thenumber Btu's of chilling required to be provided by the refrigerationsystem to chill product flowed into the freeze barrel, and therebyproduct in the freeze barrel, to within the selected temperature range,said selecting step selecting the duration of a time interval during andthroughout which the refrigeration system is to provide the determinednumber of Btu's, and including the step of ascertaining the mass flow ofrefrigerant required to be metered by the expansion valve to theevaporator to provide the determined number of Btu's, said operatingstep adjusting the speed of the compressor and the metering rate of theexpansion valve to deliver to the evaporator the ascertained mass flowof refrigerant during the selected time interval.
 49. A method as inclaim 39, wherein said operating step is performed such that therefrigerant metering rate of the expansion valve closely matches therefrigerant delivery rate of the compressor.